1. Field of the Invention
This invention relates to the field of material forming and particularly to the forming of sheet material.
2. Description of the Prior Art
The forming of sheets of metal and plastic into parts is a highly developed art. Numerous processes such as press bending, multiple slide forming, drawing, coining, spinning, roll forming, explosive forming, electromagnetic forming, and rubber pad forming have been developed for forming sheet metals as described in the American Society for Metals Handbook, Volume 4, Forming.
More recently, sheet metals have been formed under conditions which take advantage of their superplastic properties. As described in U.S. Pat. No. 3,340,101 to D. S. Fields, Jr. et al., superplastic forming is a process in which metals can be stretched into a die in a manner resembling the basic thermoforming process employed in the polymer and glass forming industries.
Except for coining and shear spinning, these prior art processes utilize predominately tensile stresses in the plane of the sheet to cause the plastic deformation of the material being formed. For example, in superplastic forming of metals or in blow forming of plastics, a gas pressure is applied to one side of the sheet of material being formed. This pressure stretches the material into a die cavity as a result of the tensile stresses created in the plane of the sheet.
Forming of sheet materials by prior art tensile forming methods is limited by tensile instability and fracture, which may occur following overall elongations of no more than 50 to 60%. Although much higher elongations can be obtained under superplastic forming conditions, the forming rates are extremely slow, requiring several hours for superplastic forming. Additionally, thickness nonuniformities develop during superplastic forming and thus produce parts having greater thickness tolerances.